The invention relates to a process for producing a decorative paper that can be impregnated with a thermosetting resin, having at the same time good printability, in particular for ink jet printing, and good resin-absorption properties. The invention also relates to such a decorative paper impregnated with a thermosetting resin. The invention finally relates to laminated decorative panels or profiles comprising the decorative paper impregnated with thermosetting resin.
Laminated decorative panels or profiles (also called “laminates”) have been employed for many years in dwellings and commercial and industrial premises. Typical applications for such laminates are floor coverings, in particular coverings imitating parquet flooring, furniture coverings, tabletops, chairs and the like.
Two main types of decorative “laminates” exist: “laminates” called high-pressure laminates and “laminates” called low-pressure laminates.
Decorative laminates called high-pressure laminates are produced from a core consisting of sheets impregnated with resin. These sheets are generally made of kraft paper and have been impregnated with a thermosetting resin, most often a phenolic resin. After having impregnated the sheets with resin, they are dried, cut up and then stacked one on top of another. The number of sheets in the stack depends on the applications and varies between 3 and 9, but may be higher. A decorative paper, also called a decor paper or decorative sheet is then placed on the stack of sheets forming the core. Such a decorative sheet is generally a sheet of paper bearing a printed or colour pattern or containing decorative particles, and it is impregnated with a thermosetting resin, for example melamine-formaldehyde resins, urea-formaldehyde resins, benzoguanamine-formaldehyde resins or unsaturated polyester resins.
In general, a protective covering sheet, called an “overlay” that has no pattern and is transparent in the final laminate, is placed on top of said decorative sheet, in order to improve the abrasion resistance of the laminate.
The stack of impregnated sheets is then placed in a laminating press, the platens of which are provided with a metal sheet giving the laminate the surface finish. The stack is then densified by heating, at a temperature of about 110° C. to 170° C., and by pressing, with a pressure of about 5.5 MPa to 11 Mpa, for approximately 25 to 60 minutes in order to obtain a unitary structure. Next, this structure is fixed to a base support, for example it is adhesively bonded to a panel of agglomerated particles, in particular agglomerated wood particles.
Decorative laminates known as low-pressure laminates are produced using only one decorative sheet impregnated with thermosetting resin, and optionally an overlay sheet, which is laminated directly onto the base support during a short cycle, the temperature being of about 160° C. to 175° C. and the pressure 1.25 MPa to 3 MPa.
The decorative paper or decorative sheet used for producing laminates is generally a paper sheet made on a paper machine. Currently marketed decorative papers are generally free from surface treatment so as in particular not to harm their impregnation speed.
These decorative papers or decorative sheets are generally used to give a special esthetic appearance to the laminated supports on which they are fixed, it being possible for this esthetic appearance to result from printing the paper with a decorative pattern.
But, this decorative pattern, which traditionally consisted in imitating the aspect of a natural material such as wood or marble, has taken many more diverse forms taking account of the demand, so as to be adapted to the customer's requirements and wishes. This increase in the variety of patterns has been on the other hand accompanied by a reduction in the quantities to be produced.
This phenomenon of the personalization of decorations and of short production runs has not been without consequences for decorative paper producers. Indeed, for the mass production of simple patterns, printing techniques such as photogravure have the advantage of printing wide web at high production rates. However, this printing method does not prove to be profitable for small production runs and moreover the printing reproduction obtained is not satisfactory for complex patterns which require high resolution.
Among the printing techniques that are sufficiently flexible for the production of small quantities on demand, ink jet printing has proved to be the technique most suited to the requirements for decor papers. Apart from the simplicity of its operating principle and relatively low production costs, ink jet printing also makes it possible to obtain better quality printing. However, use of the principle of ink jet printing in the production of decor papers remains confronted with a major difficulty associated with the method for obtaining the laminates themselves.
In the conventional process for producing laminates, the decor paper is first of all printed and then impregnated with resin and finally hot-pressed with its support at a high or low pressure. The impregnation step requires the availability of a decorative paper having a high wet strength, so as to preserve a sufficient strength after its total immersion in an aqueous resin, as well as the greatest possible capacity to absorb resin in the shortest possible time. These properties are generally obtained by using decorative papers possessing very high porosity.
But, the technique of ink jet printing rests on the principle of fixing ink to the surface of the substrate to be printed, the substrate to be printed must therefore have limited absorption so as to obtain a clear and high quality printing.
Decorative papers improved by previously coating the decor paper with a layer comprising particles for fixing the ink, have already been described. These layers are often composed of a mixture of inorganic pigments, such as calcium carbonate, kaolin, silica, and a binder of the starch, casein, latex, polyvinyl alcohol or aminoplast resin type.
Patent application EP-A-054405 relates to the improvement of the printability of decorative papers, however ink jet printing is not specifically mentioned. This document describes papers coated with a pigmented layer conventionally known in printing-writing, but in order to maintain some capacity of the paper to absorb resin, the coating weight should not exceed 10 g/m2 and the recommended weight lies between 2.5 and 3 g/m2.
Patent application EP-A-1044822 describes a decorative sheet called a finished sheet, that can be ink jet printed, formed of a paper coated with an ink-fixing layer. Said paper must be previously smoothed on one face before being coated with said ink-fixing layer. This finished sheet is then bonded with adhesive onto a panel in order to make a decorative panel. This type of pre-impregnated paper is not intended for laminates called high- or low-pressure laminates. Moreover, the paper is pre-impregnated before being coated with the ink-fixing layer, there is therefore no problem with impregnation.
Although the printing reproduction of decorative papers is appreciably improved, many disadvantages remain associated with their production method, and in particular with coating techniques.
A first disadvantage associated with the blade coating technique is that the excess of coating to be applied before scraping and the mechanical stresses of pressure and tension during contact are difficult to apply to porous fragile papers such as decor papers. This coating technique brings about many breakages during production. A second disadvantage is that the actual coating technique by contact leads to partial penetration of the coating into the paper substrate and consequently a high degradation of the capacity of the paper to absorb resin.
As the prior art reveals, production of decorative paper having a relatively high coating weight and/or preserving a high absorption capacity for producing high- or low-pressure laminates, presents many difficulties.